Well in the Caesar technical reference manual it is written than if a manifold piping is attached and the discharge of psv fills the exit and manifold piping than the force should act on the last elbow of discharge to the atmosphere whereas in case of the manifold not being filled completely than the reaction force should act on the first elbow on the exit piping. In another post in the thread mentioned below Mr.Loren has suggested that in an open system this force should act on each elbow.

http://www.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=11770&page=1#Post11770

Please clarify

Regards,
Umair